The preferred embodiments relate to electronic oscillators.
Electronic oscillators are well-known devices operable to produce an oscillating output signal. Thus, electronic oscillators are well-known for timing and synchronization and occur in numerous electronic circuits, devices, and industries. In many of these applications, various or all of the oscillator components are combined into an integrated circuit. As such, design considerations contemplate the oscillator as well as the overall integrated circuit. For example, with respect to the oscillator, key considerations are to reduce the effects of non-idealities in the oscillator and to ensure desired precision or lack of error in the output frequency. As another example, with respect to the integrated circuit, and of course the oscillator it includes, power consumption should be minimized, particularly in instances where the device for which the oscillator is operating has limited or consumable power (e.g., in battery-operated applications). As still another example, area consumption of the oscillator layout on the integrated circuit is desirably kept to a minimum.
Oscillators also are now being integrated, as are other complex analog circuits, into microcontrollers, as this trend reduces overall system cost by reducing the number of components. This trend is also opening up new markets and applications for microcontrollers as the capability of integrated analog grows. There are myriad numbers and varieties of end applications, and in this regard another key consideration for integrated analog circuits is to offer maximum flexibility to the end user. For example, many applications need an accurate integrated oscillator that may be linearly tuned over a wide frequency range. An example for such an application is a communication system with non-standard clock frequency.
One often important requirement for integrated oscillators is linearity of frequency tuning, as that enables a user to accurately choose and select frequency without having to measure the oscillator output frequency in every single chip. One prior publication in this context describes an oscillator with an 8 bit resistor option to trim the resistor so as to adjust oscillator frequency (Kunil Choe, Olivier D. Bernal, David Nuttman and Minkyu Je, “A Precision Relaxation Oscillator with a Self-Clocked Offset-Cancellation Scheme for Implantable Biomedical SoCs” in IEEE Intl. Solid-State Circuits Conf. Dig. Tech. Papers, February, 2009, pp. 402-403). Also in this context, another prior publication describes an oscillator with an 8 bit capacitor trim (A. V. Boas, et al., “A Temperature Compensated Digitally Trimmable On-Chip IC Oscillator with Low Voltage Inhibit Capability,” Proc. ISCAS, pp. 501-504, September 2004.). In A. V. Boas, et al., with an option to use an off-chip resistor, the flexibility to directly use resistor based tune/trim schemes does not exist.
Another issue arising with oscillators is that there are components that have high mismatch with significant dependence on temperature, apart from the variations in values of resistance and capacitance affecting the frequency accuracy. To improve the Temperature Coefficient (TC) of frequency drifts and close-in phase noise, chopping has been proposed for low frequency applications. Prior publications, for example, describe offset cancellation techniques applied to an RC oscillator, but for low (32 kHz) and moderate frequencies (3 MHz) (see, e.g., Keng-Jan. Hsiao, “A 32.4 ppm/° C. 3.2-1.6V self-chopped relaxation oscillator with adaptive supply generation,” Dig. Symp. VLSI Circuits, pp. 14-15, June 2012.; see also, above-introduced Kunil Choe, Olivier D. Bernal, David Nuttman and Minkyu Je, “A Precision Relaxation Oscillator with a Self-Clocked Offset-Cancellation Scheme for Implantable Biomedical SoCs” in IEEE Intl. Solid-State Circuits Conf. Dig. Tech. Papers, February, 2009, pp. 402-403.)
Given the preceding, the present inventors have identified improvements to the prior art, as are further detailed below.